We propose studies to examine those interactions between poliovirus and its host cell which result in selective interference with protein synthesis directed by cellular messenger RNA. Following infection of HeLa cells with poliovirus, there is a rapid and marked inhibition of host cell protein synthesis, which results in disaggregation of host cell polyribosome structures prior to the formation of poliovirus-specific polysomes and translation of viral mRNA. The untranslated cellular mRNA remains structurally intact, polyadenylated, capped and methylated, and has shown to function normally in vitro after extraction from infected cells. Translation of this mRNA in vivo, however, is blocked at the step of initiation complex formation. Preparations of initiation factors from infected and uninfected cells shown an mRNA discriminating activity which results in the failure of infected cell initiation factors to function in translation of host cell proteins, although the preparation is active for stimulating translation of viral proteins. We are examining both cellular and viral protein synthesis in vitro to determine the difference in requirements for host cell and viral protein synthesis and to analyze the factor causing restriction. We hope to understand the mechanisms by which selectivity for different mRNAs can be imposed on protein synthesis.